Calcium carbonate which is produced by chemical means is often referred to as precipitated calcium carbonate. Precipitated calcium carbonate has various advantages over the mined, natural calcium carbonate materials. For example, the precipitated calcium carbonate is pure and exhibits higher brightness and opacity, particularly when used as a filler in paper. Additionally, the particle size, particle shape and other physical properties of precipitated calcium carbonate may be controlled by reaction conditions in order to adapt the precipitated product for various uses. Generally, calcium carbonate may be precipitated as either aragonite, the metastable form of calcium carbonate, or calcite, the stable form of calcium carbonate, or mixtures thereof, depending on reaction conditions. Generally, calcite is superior to argonite, particularly in alkaline sized paper properties.
Naturally occurring, mined calcium carbonate may be subjected to a heat treatment such as calcination to produce quick lime (calcium oxide). A well known method for producing calcium carbonate using quick lime is as a byproduct in the production of caustic soda from soda ash. The reaction proceeds according to the following equation: EQU CaO+H.sub.2 O+Na.sub.2 CO.sub.3 --NaOH+CaCO.sub.3
Such methods are disclosed in the Church et al. U.S. Pat. No. 1,862,176, wherein the CaCO.sub.3 wet cake is treated with additional Na.sub.2 CO.sub.3, CO.sub.2, and CaCl.sub.2 to remove alkali and in the Brooks et al U.S. Pat. No. 2,062,255, wherein an excess of lime is used in the causticization reaction and the resultant wet cake is treated with additional soda ash. The Casciani et al. U.S. Pat. No. 3,268,387 and the Miller U.S. Pat. No. 2,979,380 also disclose methods for producing precipitated calcium carbonate using causticization reactions. Many commercial caustic plants, however, have discontinued these methods in favor of producing caustic by the electrolysis of common salt, whereby chlorine and hydrogen are also produced
Alternatively, it is well known to slake the quick lime in water to form a calcium hydroxide slurry through which carbon dioxide is then bubbled to precipitate calcium carbonate according to the following reactions: EQU CaO+H.sub.2 O.fwdarw.Ca(OH).sub.2 EQU Ca(OH).sub.2 +CO.sub.2 .fwdarw.CaCO.sub.3 +H.sub.2 O
Methods for carbonating calcium hydroxide slurries are disclosed in the Rafton et al. U.S. Pat. No. 2,058,503 and the Statham et al. U.S. Pat. No. 2,081,112. Generally, carbon dioxide is bubbled through a vigorously agitated calcium hydroxide slurry solution. Additional carbonation methods are disclosed in the Waldeck U.S. Pat. No. 3,320,026 and the Vanderheiden U.S. Pat. No. 4,367,207.
A disadvantage of the carbonation methods known in the art is that the carbon dioxide utilization efficiency, at best, approaches only about 60%, and it is often difficult and costly to recover the unused carbon dioxide. Accordingly, a need exists to develop a carbonation method for producing precipitated calcium carbonate which provides an improved carbon dioxide utilization efficiency.